Froth flotation is one of the primary solid-solid separation processes for fine particles. The process has been widely practiced for a century in the mining industry for concentrating valuable minerals such as copper, lead, zinc, phosphate, precious metals, and coal, among others. Typically the froth flotation process has been developed to work in water with air as the froth generating gas, however other liquid and gas combinations can be used.
In the froth flotation process, one or more specific particulate constituents of a slurry or suspension of finely dispersed particles attach to gas bubbles and are subsequently separated from the other constituents of the slurry or suspension. The froth flotation process exploits the wettability differences of the particles to be separated. Differences in the wettability among solid minerals particles can be natural, or can be induced by the use of chemical additives. The buoyancy of the bubble/particle aggregate, formed by the adhesion of the gas bubble to a particle in the slurry, is such that it rises to the surface of the flotation vessel where it is separated from the remaining particulate constituents which remain suspended in the aqueous phase of the suspension.
In a typical operation, a mineral ore or a coal is pulverized to a fineness suitable for liberating undesired components from the valuable constituent. After the pulverization, the material is conditioned with reagents, known as collectors and surfactants, to render the valuable constituent hydrophobic. In some instances, the contaminant or gangue is rendered hydrophobic. In the case of coal, hydrocarbon oils are often used as collectors. After conditioning, the material is carried by a feed stream to a liquid containing flotation column and the feed stream injects directly into the liquid at a height typically somewhere in the upper third of the column. At the same time gas bubbles are introduced at the bottom of the column. Once introduced into the flotation column, the material in the feed stream moves downward in the flotation column liquid while the gas bubbles move upward, producing counter-current flow. This countercurrent feeding arrangement promotes an interceptional collision between the particles in the feed stream and the gas bubbles, and produces a collection zone in the flotation column. In the collection zone, of the particles that collide with the gas bubbles, those that are sufficiently hydrophobic are collected by the bubbles and rise through the collection zone as bubble-particle aggregates. Above the collection zone, the bubble-particle aggregates gather and form a froth zone. The bubble-particle aggregates move upward through the froth zone and exit the flotation column at a froth overflow, where the froth is broken and the particles are liberated from the bubble-particle aggregates. In this manner, generally speaking, feed streams containing both hydrophobic and hydrophillac particles may be separated, with generally the more hydrophobic particles reporting to the froth and the more hydrophilic particles exiting the column through the tailings port.
The effectiveness of these methods is often measured in terms of recovery and grade. Recovery refers to the amount of the valuable constituent in the product stream relative to the amount of that constituent in the feed stream. Grade refers to the concentration of the valuable constituent in the product stream relative to the concentration of all the constituents in the concentrate stream. Higher recoveries and grades are desired in a separation process.
Conventional methods suffer when the feed stream is comprised of particles of varying hydrophobicity, and the goal of the process is separation of particles having the strongest hydrophobicity. One example is an application intended to separate coal from a feed stream having particles comprised of both coal and low grade ash and sulfur. A particle that contains as little as 10% coal on its surface, and thus represents a high ash content particle, has a good chance to report to the flotation product as a result of bubble attachment to the coal portion of the particle surface. As a result, the bubble-particle aggregates which move upward through the froth zone and exit at the froth overflow includes a higher than desired content of the low grade ash and sulfur when the froth is broken.
It is known that one method of reducing the quantity of high ash content particles reporting to the froth breaker is through the mechanism of internal reflux. As the bubble-particle aggregates proceed upward through the flotation column, interstitial liquid between the bubbles drains away and the bubbles begin to coalesce. This coalescence liberates formerly attached particles and reduces the bubble surface area available for reattachment. Competition for the reduced bubble surface area favors the more hydrophobic particles, and the quantity of lower hydrophobic, high ash content particles reporting to the froth breaker is reduced. Some flotation columns are designed to intentionally increase the extent of bubble coalescence that occurs, thereby increasing the internal reflux and reducing the quantity of lower hydrophobic particles in the end product. However, control of the process is difficult when the extent of bubble coalescence is a direct function of original separation column design, and the operating environment deviates significantly from the design environment. In such cases, varying combinations of wash-water rate, gas rate, feed rate, and other operating parameters must be attempted in order to obtain an end product possessing the desired recovery and grade. It would be advantageous if control over end-product recovery and grade was available through adjustment of more discrete operating parameters.
Another method of increasing the grade involves the introduction of higher hydrophobicity material into the froth zone following particle collection by feed injection into the liquid zone. See e.g., Honaker et al, “Selective detachment process in column flotation froth,” Minerals Engineering 19 (2006), and see Ata et al, “Collection of hydrophobic particles in the froth phase,” Int. Jour. Miner. Process. 64 (2002). In these methods, a foreign material having high hydrophobicity is added directly to a particle-laden froth, so that during bubble coalescence the reduced bubble surface area will favor attachment of the added foreign material, and less hydrophobic materials will be selectively detached. This methodology can increase the grade of the end-product feed stream with respect to the valuable, hydrophobic constituent, however by the nature of the process the end-product feed stream also contains the foreign material. This foreign material must then be subsequently removed, adding additional operational steps in addition to the prerequisite supply of the foreign material itself. It would be advantageous if this selectivity mechanism could be employed in a manner obviating introduction of foreign material, so that the need for a supply and subsequent removal could be eliminated. Further, it would be advantageous if the selectivity mechanism did not employ bubble coalescence as a requirement, such that more stable froths could be utilized.
Additionally, the interstitial liquid existing between bubbles in the froth zone contains a significant amount of the less hydrophobic or even hydrophilic material. This material may also be carried over to the froth breaker with the interstitial liquid, producing a higher than desired amount of low grade materials in the end product. To combat this particular problem, clean wash-water is applied to the top of the froth. The clean wash-water generally flows downward through the froth and displaces the interstitial water containing the less hydrophobic and hydrophilic materials, so that those materials are washed from the froth and removed as a tailings stream. This is a widely practiced and generally effective technique in industry, however the necessity for a continuous source of clean wash-water is a significant operational requirement. Wash-water rates of 3-5 gpm/ft2 are typical for commercial installations. Additionally, optimization can be difficult. Excessive wash-water flows should be avoided because excessive wash-water passing downward through a column creates an undesirable reduction in the slurry residence time in the froth, and a possible reduction in recovery. High water additions may also destabilize the froth by stripping surfactant from the surface of the bubbles, and may act to decrease product grade by increasing axial froth mixing, reducing wash-water effectiveness. It may also produce excessively dilute column tailings products which are difficult to thicken. It would be advantageous if the reliance on wash-water could be reduced or eliminated, and lower hydrophobicity and hydrophilic material could be removed by interstitial water displacement without reliance on a clean wash-water supply.
The typical flotation column, as discussed supra, contains a froth zone floating on a liquid zone, and injects a feed slurry into the liquid zone. Bubbles are generated and introduced into the liquid zone with the feed, and collision between the bubbles and the hydrophobic particles is relied upon to create the necessary bubble-particle aggregate which then reports to the froth zone. This arrangement levies a number of significant requirements. Numerous efforts are aimed toward increasing the probability of bubble-particle collision, reducing the degree to which hydrophobic particles are sheared off as bubbles transition from liquid to froth, and maintaining an optimum ratio of bubble size to particle size. The latter in particular places significant operational constraints on a separation process, as a bubble size too large relative to the hydrophobic particle results in the particle sweeping around the bubble, rather than colliding, and a bubble size too small relative to the hydrophobic particle may have insufficient buoyancy with which to carry the particle to the froth zone. In either case, a significant amount of the hydrophobic material reports to the tailings stream. These issues can be somewhat mitigated by injection of the feed stream directly into the froth zone, as opposed to the liquid. Particle collection rates using froth injection are generally higher than injection into the liquid zone due to short collision path lengths and high interfacial bubble area per unit volume in the froth. Additionally, turbulence in the froth is low, reducing the tendency for attached particles to break away from bubbles, and particles may adhere to several bubbles, rather than just one as typically occurs in liquid zone bubble-particle collisions. As a result, froth injection is particularly effective for coarse particles, allowing capture of particles 5-10 times the upper limiting size for liquid injection columns. This directly impacts the crushing and grinding requirements prior to introduction of the particles into the separation column. Fine particle collection is also enhanced, as the particles are introduced to the bubble bed and the tendency of these low inertia particles to follow bubble streamlines and avoid capture is mitigated
Such froth injection systems are known. See, e.g., Schultz et al, “The flotation column as a froth separator,” Mining Engr. 1450 (1991). However, the main drawback to froth injection is the poor selectivity among particles of varying hydrophobicity. Some finite recovery of small hydrophilic particles is always observed in froth injection. See Nguyen et al, Colloidal science of flotation, Marcel Dekker, New York (2004). It would be advantageous to provide a method whereby froth injection is utilized for a high degree of particle collection from a feed, but carryover of lower hydrophobic or hydrophilic particles to the froth overflow could be substantially minimized.
Accordingly, it is an object of this disclosure to provide a method of operating a flotation column where froth injection is utilized for a high degree of particle collection with reduced carryover of lower hydrophobic or hydrophilic particles to the froth overflow.
Further, it is an object of this disclosure to provide a method of operating a flotation column where froth injection is utilized and control over end-product recovery and grade is available through adjustment of a limited number of discrete operating parameters.
Further, it is an object of this disclosure to provide a method of operating a flotation column where froth injection is utilized and lower hydrophobic and hydrophilic material is removed by interstitial water displacement with reduced or eliminated reliance on a clean wash-water supply.
Further, it is an object of this disclosure to provide a method of operating a flotation column allowing capture of coarse particles beyond the upper limiting size for liquid injection columns.
Further, it is an object of this disclosure to provide a method of operating a flotation column allowing capture of fine particle by introducing the fine particles directly to a bubble bed, mitigating the tendency of the low inertia particles to follow bubble streamlines and avoid capture.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.